1. Field of the Invention
This invention relates to thin film disks. More specifically, the invention relates to methods and systems for measuring disk overcoat layers.
2. Description of the Related Art
Thin film disks, such as disk drives, are used in a variety of applications including computers, car stereos, vending machines, media players, and automated teller machines (ATMs). In the foreseeable future, such disks will be introduced into additional machines and environments due to greater storage capacities, smaller footprints, and decreased prices. As thin film disks become more pervasive, the ability to ensure high performance becomes essential to delivering competitive products.
An integral part of ensuring high performance relates directly to the thickness of the overcoat layer on the thin film disks. The thickness and uniformity of the overcoat layer have a direct effect on the recording density achievable on magnetic media. Consequently, precise measurement of the overcoat thickness is critical to successfully manufacturing thin film disks.
One prior art method to measure overcoat layer thickness is known as ellipsometry. Ellipsometry is a highly accurate measurement technique that uses polarized light to characterize thin films, surfaces, and material microstructure. Ellipsometry uses a beam of incident light of known polarization that interacts with the material of interest, resulting in changes in the polarization of the light. The changes in polarization are measured by analyzing the light reflected from the sample.
Through the analysis of the reflected polarization, ellipsometry can yield information about layers that are thinner than the wavelength of the light itself, down to a single atomic layer or less. Depending on what is already known about the sample, the technique can probe a range of properties including the layer thickness, morphology, or chemical composition.
While very useful in measuring overcoat layer thickness, ellipsometry is dependent on a knowledge of the optical properties (such as the index of refraction and extinction coefficient) of the surface on which the overcoat layer resides. Inaccurate data or estimates due to variations in material composition and manufacturing variance may reduce the accuracy of thickness measurements for the overcoat layer.
FIG. 1 illustrates one embodiment of a thin film disk 100 with one or more base layers 110 including a substrate 120 and an overcoat layer 130. In practice, the optical properties of the base layers 110 vary due to differences in material composition, material thickness, material density, and the like. Variation in the properties of the base layers can result in measurement errors and can inhibit accurate determination of the thickness of the overcoat layer. In short, improving the performance of thin film disks 100 is impeded by measurement errors resulting from an inability to account for the variation in optical properties of the base layers 110.
From the foregoing discussion, it should be apparent that a need exists for a method and system for measuring overcoat layer thickness on a thin film disk with increased accuracy. Beneficially, such a method and system would facilitate obtaining improved thickness measurements for overcoat layers on thin film disks and would facilitate optimization of the processes associated with manufacturing thin film disks.